Field of the Invention
The invention relates to a diaphragm carburetor, in particular for use in a two-stroke engine, including a diaphragm-controlled closed-loop control chamber for fuel that communicates through a number of fuel conduits with an aspiration conduit for air.
A diaphragm carburetor is used to adjust the combustion mixture of air as well as fuel finely dispersed in the air, which is required in the applicable operating state of an internal combustion engine, especially in the idling, partial-load and full-load operating modes, in a mixture ratio that is optimized for combustion. Typically, diaphragm carburetors are used on one hand in small tools, such as a power chainsaw. On the other hand, diaphragm carburetors constructed for comparatively large motors, such as motors with a displacement of more than 300 cm.sup.3, are often used in motor boats and especially in so-called jet skis.
In a diaphragm carburetor known from German Published, Non-Prosecuted Patent Application DE 44 09 887 A1, a further conduit system is present in addition to a main channel for the outlet of fuel into an aspiration conduit. That conduit system includes an emulsion chamber, which communicates with a closed-loop control chamber for fuel delivery and from which a bypass conduit and an idling conduit branch off. The idling conduit discharges through its outlet opening into a partial chamber of the aspiration conduit oriented toward the motor, while the bypass conduit discharges into a partial chamber remote from the motor. The two partial chambers are separated in the idling mode by a rotary throttle valve disposed in the aspiration conduit, which nearly closes the aspiration conduit in the idling mode. When the motor is running, a negative pressure develops in the partial chamber toward the motor, while atmospheric pressure prevails in the partial chamber remote from the motor. As a consequence of that pressure difference between the two partial chambers, air is aspirated through the bypass conduit and mixes with fuel in the emulsion chamber. The fuel-air mixture is aspirated through the idling conduit and delivered to the motor.
Upon acceleration with the attendant tilting of the throttle valve, the outlet openings of the bypass conduit enter the negative pressure range. That causes a reversal of the flow direction in the bypass conduit and the emergence of fuel through the bypass openings into the aspiration conduit. Therefore, in partial-load operation, the bypass conduit serves to deliver fuel and thus performs the function of a partial load conduit. However, until pure fuel, that is fuel unmixed with air, can be dispensed into the aspiration conduit, the air located in the emulsion chamber and in the bypass conduit must first be positively displaced. Upon an abrupt opening of the throttle valve, that leads to delayed acceleration of the motor. In tools, such as a power chainsaw, that effect is of only slight significance. In motor boats and especially jet skis, poor motor pickup is extremely undesirable.
In the prior art which is disclosed, for instance, in German Published, Non-Prosecuted Patent Application DE 196 04 553 A1, an acceleration pump in the form of an additional diaphragm pump which is actuated through a cam coupled to the pivot axis of the throttle valve, is used to overcome that problem. The acceleration pump sends a fuel surge into the carburetor upon a complete, sudden opening of the throttle valve. The fuel surge suffices to compensate for the aforementioned delays in acceleration. Fuel supply subsequently takes place essentially through the main channel. However, the acceleration pump causes a great deviation in the fuel-air mixture ratio from the optimal range for combustion, because of the sudden fuel surge during the acceleration event. That deviation is expressed in a worsening of motor efficiency. On one hand, it makes for high fuel consumption and on the other hand high pollutant emissions. Integrating an additional acceleration pump into the carburetor system also requires complicated, cost-intensive planning of the construction.
A further problem arises when an acceleration pump is used, particularly in a jet ski. In harbor areas, jet skis must run at idling rpm and thus at a walking pace. Trips of that kind can last up to 15 minutes. During that period, the combustion mixture collects in the crankcase. Upon a sudden increase in motor rpm, the mixture is activated and is aspirated into the combustion chamber along with the fuel injected by the acceleration pump. The result can be unstable operation and in an extreme case even failure of the motor.